1. Field of the Invention
The present invention relates to a method of finely crushing particles of material in an impact mill, wherein at least one rotor provided with at least one impact surface projects the particles against stationary impact surfaces in the mill, the comminution taking place substantially in vacuum.
The invention also relates to an apparatus for performing the method.
2. Description of the Prior Art
Mechanical comminution of solid particles may be classified according to the manner of operation of the apparatus that performs the crushing. Crushing or milling may be performed according to two main principles. Either with pressure that ruptures the particles, as for instance by means of jaw crushers, pendulum mills, tower mills, roller mills and the like, or with kinetic energy that causes bursting of the particles. Typical examples of the latter category are hammer mills, pin mills and jet mills.
There is a great demand on the market for finely crushed material especially in the filler industry as fillers e.g. in the plastics and paper industries. Another large product area is finely milled quartz and feldspar for the ceramic industry. Todays technology for producing fine grained materials are very energy and cost demanding, especially when narrow tolerances in grain size and particle sizes below 30.mu. are desired. Sizes down to 45.mu. may be obtained with ball mills, but since the grinding balls usually consist of iron it is not possible to obtain iron-free milling. This type of milling therefore is not useable for materials which are to be used as whitening agents in the paper and in plastic industry, since even small proportions of iron destroy the whiteness. In order to arrive at particle sizes of 20.mu., either conventional milling and air stream separation or a jet mill is used. The problem with air-separated material is that too much of too coarse fractions will be included, which is mostly not acceptable. With jet mills it is possible to arrive at particle sizes around 10.mu. and even smaller, but the jet mill has a low efficiency and a very high energy consumption (350-700 kWh/ton).
The amount of energy consumed in crushing by means of kinetic energy or impact energy may be divided into three main groups, viz. energy which is consumed for elastic deformation of the grains or particles and which is lost, supplied energy which bursts the grains, and energy for the operation of surrounding equipment etc. At low velocities, a substantial portion of the supplied energy will be used up for the elastic deformation of the particles, which gives a low efficency. Furthermore, the elastic deformation will cause the particles to bounce instead of bursting which will cause heavy wear. At high velocities, on the other hand, the energy consumed for elastic deformation amounts only to a small portion of the total energy, whereas the high impact energy, if it can be applied as "instantaneously" as possible, gives very high stress concentrations and provides an efficient crushing. This means that a high particle velocity and instantaneous impact force should be chosen. The jet mill which has proved to be especially suitable for crushing or milling into fractions with very small grains utilizes a high particle velocity and uses pressurized air for accelerating the grains to about 100 m/s, and the high velocity grains collide with other grains resulting in the grains bursting each other. However, due to the very high energy consumption per ton, low efficiency and very high production costs, the jet mill has so far only been used in "exclusive" connections, for instance in the chemical industry and the pharmaceutical industry.
It has also been suggested (German patent specification No. 387.995) to use disintegrators or stamp mills, the milling tools of which operate in an air-void space in order to obtain a fine grinding, preferably together with a dispersing agent.